For many years, those working in the glass art have attempted to solve the problem of reversible opacification as it applies to glazings. By "reversible opacification", applicants mean the process by which a transparent or semi-transparent glazing is rendered opaque and subsequently rendered at least partially transparent once again. The prior research efforts in this area have been directed to experiments dealing with various coating materials used to control the intensity or degree of the light radiation passing through the glazing.
One tangible result of these experiments is the so-called "photochromic" glasses which are currently marketed, particularly for the production of corrective eyewear. These glasses are at least partially opacified by the effect of ultraviolet radiation. Once the exposure to this radiation is terminated, however, they gradually revert to a transparent state.
Photochromic systems of this type are, however, described as "passive" in the sense that the user can not control by his actions the degree of opacification which the glass undergoes. Further, these photochromic glazings cannot be rendered opaque in a "low light" environment, such as that which is encountered outdoors at night. Clearly, therefore coatings of this type can not be utilized to bar all light from passing through the glazing.
In order to overcome these difficulties, efforts have been directed toward producing a "dynamic" variable transmission glazing, i.e., one wherein the degree of opacification may be deliberately controlled in any optical environment by the user. In this respect, the applicants have undertaken to investigate the field of electrochromic systems, wherein a color change may be intentionally obtained within a coated glazing due to the effect of an electric current passed through the coated substrate.
Electrochromic systems have previously been proposed for use, for example, in batteries, wherein their electrical properties are of interest, and in windows (i.e., glazings) wherein their optical properties have been investigated. The optical application for these materials is more particularly the object of the present application. In these optical applications, the system may be operated either in a reflection mode, wherein mirrors and alphanumeric display devices are required, or in a transmission mode, such as in the case of glazings utilized by the construction industry for installation in newly constructed buildings. It should be noted, however, that while, on the surface, systems utilizing the reflection and the transmission mode appear to be very similar, transmission glazings are subjected to far more rigorous demands than those utilizing the reflection process.
These requirements are detailed below and, until the development of the present invention, those working in this field had not been able to sufficiently overcome them to permit the development of a practical transmission glazing, even at the pilot plant level. In the following description applicants will disclose mainly electrochromic systems which function by transmission, with the understanding that nothing prevents the same materials and the same processes from being used for systems functioning by reflection.
The electrochromic systems designed by applicants and disclosed herein have overcome the difficulties previously encountered by others working in the field. The system comprises: two support plates, of glass for example, each coated with an electroconductive deposit acting as an electrode. The plates are separated by at least a layer of an electrochromic material and a layer of an electrolyte. The most frequently used electrochromic material is colorless tungsten oxide (WO.sub.3) which becomes midnight blue in color, especially in the presence of protons.
Other electrochromic materials, particularly those with an iridium oxide base, form different colors when exposed to protons. When an electrochromic layer having a tungsten oxide base, which base constitutes an ion transfer layer, is used, numerous materials may be used as an electrolyte. None of these materials, however, have been determined to be fully satisfactory, at least for use in large electrochromic systems.
The first electrochromic systems were developed using a liquid electrolytic layer, which comprised a liquid solution of, for example, a strong acid. However, besides attacking the other layers almost immediately upon contact, these liquid electrolytes were further found to be very delicate to incorporate since their use requires very special care in the vicinity of the seals that insulate the system. In practice, therefore, these liquid electrolytes are used only for smaller electrochromic systems such as alphanumeric displays.
Systems known as "all solid" systems have also been previously proposed. These electrochromic systems may be classified as a function of the thickness of the electrolyte. Electrolytes which are deposited in "thick" layers, i.e., in thicknesses of generally more than 1 micron, for example, frequently utilize a base of mineral acids such as uranyl phosphoric acid. It has further been proposed to use liquid electrolytes of the polymer type for this purpose, particularly lithium salts buried in a porous medium. The main difficulties encountered in using these thick electrolyte layers are, however, (1) the appearance of diffraction zones which harm the optical quality of the glazing and (2) the toxicity inherent in the polymer electrolytes used, particularly salts of lithium, so that the large-scale use of this type of electrochromic glazing would be particularly burdensome.
Finally, there are "all solid" systems which utilize a "thin" electrolytic layer, measuring on the order of about 150 nanometers. Ion conductive glasses having this type of layer are often used. They comprise a thin dielectric layer of silica or magnesium fluoride, for example, doped with water molecules. This gives the layer a proton conductivity. These dielectrics are deposited under a vacuum, according to procedures well-known in the art and they are currently used to produce, for example, gold and silver glazings.
It has been observed by the applicants, however, that discontinuities appear within these "thin" layers as soon as the surface area to be coated exceeds about 10.times.10 cm.sup.2. These discontinuities, which occur due to the very fine deposited thickness, lead to the formation of "holes or disruptions" in the coating. These holes may cause a short circuit, leading to an internal discharge of current within the device and a rapid spontaneous discoloration of the system.